Microwave filter comprising absorbing structures for removing suprious wave energy



Nov. 14, 1967 v, o, MET 3,353,123

MICROWAVE FILTER COMPRISING ABSORBING STRUCTURES FOR REMOVING SPURIOUSWAVE ENERGY Original Filed March 51, 1959 5 Sheets-Sheet 1 EIELJ V/z vx?0; 4/57 IN V EN T 0R.

Nov. 14, 1967 v. o, MET 3,353,123

MICROWAVE FILTER COMPRISING ABSORBING STRUCTURES FOR REMOVING SPURIOUSWAVE ENERGY Original Filed March 31, 1959 5 Sheets-Sheet 2 mlssef/m/ zas: v

2 a 4 w 5 6 7 M a 2455 ad/U0 870, BJA/fi 407 554455 ,e/az/i 5434455 FIELMM vz 0. M27

'INVENTOR.

m wax 3,353,123 UCTURES Nov. 14, 1967 v. o. MET

MICROWAVE FILTER COMPRISING ABSORBING STR FOR REMOVING SPURIOUS WAVEENERGY Original Filed March 51, 1959 5 Sheets-Sheet 3 0. MAY

1N VEN TOR.

Nov. 14, 1967 v. o. MET 3,3

MICROWAVE FILTER COMPRISING ABSORBING STRUCTURES FOR REMOVING SPURIOUSWAVE ENERGY Original Filed March 51, 1959 5 Sheets-Sheet 4 I N V EN TOR.w%n

v. o. MET 3,353,123 MICROWAVE FILTER COMPRISING ABSORBING STRUCTURESNov. 14, 1967 FOR REMOVING SPURIOUS WAVE ENERGY Original Filed March 31,1959 5 Sheets-Sheet 5 wz az 0. 44A;

United States Patent 3,353,123 MICRUWAVE FILTER COMPRISING ABSORB- INGSTRUCTURES FOR REMOVING SPURI- OUS WAVE ENERGY Vilttor 0. Met, PaloAlto, Calif., assignor to General Electric Company, a corporation of NewYork Continuation of application Ser. No. 184,626, Mar. 7, 1962. Thisapplication Sept. 1, 1965, Ser. No. 489,783 20 Claims. ((31. 333-73)This application is a continuation of application, Ser. No. 184,626,filed Mar. 7, 1962, now abandoned.

This invention relates to filters for high frequency electromagneticwaves and has for its object the provision of a device for frequencyselective energy absorption in the microwave region.

The increased use of the microwave spectrum renders problems ofinterference more and more acute. Receivers are continually being mademore sensitive and the power output of the transmitters available isbeing increased. Consequently, the provision of frequency selectivemicrowave filters both for the transmitters as well as receiver channelsis becoming a necessity. This is particularly true since microwave powergenerators, for example, magnetron oscillators, produce harmonic powerin appreciable quantities. Thus, direct interference with systemsoperating at harmonic frequencies of the transmitter results when theharmonics are not suppressed at the source. An example is interferenceof the third harmonic of an S-band (2.6- 3.95 kilomegacycles)transmitter with X-band (SJ-12.40 k.m.c.) systems.

Filtering the output of high power microwave sources is particularlydifficult since such sources may be severely damaged by reflectedenergy, possibly energy of the higher harmonic frequencies, and becausethe filter must have high power handling capabilities due to itsposition relative to the generator.

Two of the most common types of microwave filter in use are the ladderor series resonator filter and the parallel resonator or branch filter.In the ladder type microwave filter, the input and output waveguides ofthe system are interconnected by a series of resonators coupled intandem one to another so that a signal of any frequency must passthrough all resonators in going from the input to the output. In theparallel resonator or branch filter, the input and output waveguides areconnected by a number of resonators, each coupled directly to both inputand output guides. Signal components of different frequencies can passfrom the input to the output guides through different resonators.

Neither of these filter types has proven satisfactory for use with highpower microwave frequency sources since resonant type microwave filtersare reflective. Further, the configuration of resonance elementsemployed in such filters results in the existence of regions of highelectric field strength. Thus, unless the filters employing suchelements are evacuated, their power handling capabilities are not high.An additional disadvantage of the filters employing resonant sections isthat the filter becomes complex to the point of impracticability if theharmonics of the unwanted modes launched in the system by the microwavegenerator are to be suppressed. This is true since each mode launched inthe system requires special attention and eventually leads to anadditional filter section.

Accordingly, it is an object of the present invention to provide afrequency selective filter for use in microwave frequencies whereinpower at unwanted frequencies and modes is absorbed rather thanreflected.

In carrying out the present invention, a frequency selective filter forenergy in the microwave spectrum is provided by coupling an absorptivestructure to the main ice waveguide through a suitably chosen apertureor apertures such that the fundamental is essentially unperturbed butharmonics are dissipated in the absorptive structure which is beyond cutoff for the fundamental.

The novel features which are believed to be characteristic of theinvention are set forth in the appended claims. The invention itself,however, both as to its organization and method of operation, togetherwith further objects and advantages thereof, may best be understood byreference to the following description taken in connection with theaccompanying drawings in which:

FIGURE 1 is a partially broken away perspective view of a filter ofgeneralized shape which is used to illustrate principles of the presentinvention;

FIGURE 2 is a partially broken away perspective view of a particularembodiment of a filter employing the present invention;

FIGURE 3 is a partially broken away, central, longitudinal, verticalsection through the filter of FIGURE 2;

FIGURE 4 is a graph showing the Voltage Standing Wave Radio (VSWR) andinsertion loss in decibels plotted along the axis of the ordinates andfrequency in kilomegacycles (k.m.c.) illustrating the transmissioncharacteristics of the absorption type filter of FIGURES 2 and 3;

FIGURE 5 is a perspective view of an absorption structure similar to theperiodic absorptive structure illustrated in FIGURES 2 and 3 which maybe utilized in connection with a waveguide segment to provide anabsorptive type filter;

FIGURES 6, 7, 8 and 9 are partially broken away perspective viewsillustrating still other embodiments of absorptive type filtersemploying principles of the present invention wherein both the energywhich is transmitted and that which is absorbed is propagated inparallel guides;

FIGURE 10 shows a central, vertical, longitudinal section of a portionof a filter construction using a double ridged waveguide in accordancewith principles of the.

present invention;

FIGURE 11 shows a sectional view of the filter of FIGURE 10 taken alonglines 1111 of that figure;

FIGURE 12 is a partially broken away perspective view of another filterconstructed from a double ridged waveguide and utilizing concepts of thepresent invention;

FIGURE 13 is a central, vertical, longitudinal section taken through asection of a coaxial absorptive type filter embodying the concepts ofthe present invention;

FIGURE 14 is a sectional view of the coaxial filter of FIGURE 13 takenalong section lines 14-44;

FIGURE 15 is a central, vertical, longitudinal section taken throughanother embodiment of a coaxial absorptive type filter utilizingconcepts of the present invention; and

FIGURE 16 is a sectional view through the coaxial filter of FIGURE 1;5taken along section lines 16-16 of that figure.

FIGURE 17 is a partially broken away perspective view of another type ofabsorptive filter employing the concepts of the present invention.

FIGURE 1 is an illustration of a simple version of a frequency selectiveabsorptive filter presented for the purpose of illustrating the basicconcept of the invention. The simplified filter of this figure includesa main waveguide or waveguide segment 10 of rectangular cross sectionwhich is inserted in a waveguide system in such a manner that thefundamental frequency and mode and harmonics of the fundamentalfrequency with accompanying modes which appear in the waveguide systemmust be propagated through the waveguide segment 10. In order to removeenergy of unwanted frequencies and modes from the main waveguide segment10, an auxiliary waveguide section or absorber 11 is coupled to the mainwaveguide segment by means of a rectangular coupling aperture 12 in theupper broad wall 13 of the main guide and centrally located betweennarrow side walls 14. As illustrated, the absorber 11 has a rectangularcross section of smaller dimensions than the main waveguide segment 10and is placed adjacent to the upper board wall 13 of the main waveguidesegment 10 with its longitudinal axis perpendicular to and intersectingthe longitudinal axis of the main waveguide segment 10 and its narrowwalls 15 parallel to the narrow side wall 14 of the main waveguidesegment. Energy entering theabsorber 11 is dissipated in a lowreflective wide band load 16 which is illustrated schematically inFIGURE 1 by the conventional conductor to ground symbol.

The frequency selectivity of the filter is a result of designing theabsorber waveguide section 15 to exhibit distinct cutolf properties at afrequency which denotes the upper end of the pass band of the filter andselecting the size and shape of the coupling aperture 12 to match theelectric fields of the undesired electromagnetic waves propagating inthe main waveguide 10 to the absorber section 16. As is well known inthe art, the cut off properties of the absorber waveguide section 15 andthe coupling aperture 12 are determined by the cross sectionaldimensions and shape. For a discussion of design criteria to apply, seePrinciples and Applications of Waveguide Transmission by George C.Southworth, Van Nostrand Company, Inc., Princeton, N.I., 1950, andparticularly Chapter V which starts on page 97. Below the cutofffrequency of the absorber 11,the coupling aperture 12 represents areactive loading of the main guide, while at frequencies above cut olf,energy enters the absorber 11 and is dissipated in its load 16. In thismanner, the fundamental frequencies and mode or modes which are desiredin the system pass through the main waveguide segment 10 substantiallyunaffected. That is, they are neither.reflected nor attenuated in anysubstantial amount, whereas frequencies and modes which arenot desiredin the system, are coupled into the absorber segment 11 and absorbedtherein, thus eliminating reflection of these frequencies back into themain system.

It will, of course, be recognized that the rectangular waveguidesillustrated in FIGURE 1 are utilized for illustrative purposes and theprinciples may be applied to waveguides of any configuration. Further,it will be recognized that in practice a single absorber segmentgenerally will not be sufficient to render any appreciable amount ofenergy dissipation in the desired stop band. Consequently, it is usuallynecessary to construct the absorber from an array of individual absorberwaveguide sections. It is most advantageous to utilize a periodic arrayof such absorber sections. If it is possible to design each couplingaperture so that there is a perfect reflectionless match between themain waveguide and each absorptive waveguide section for energy offrequencies in the stop band of the filter, the arrangement of theabsorptive sections in the absorber or absorptive array is not criticalwith respect to reflective properties. However, each coupling aperturerepresents a reactive loading of the main waveguide, and the effects ofthe reactances may be eliminated (tuned out) best and most completely ifthe. coupling apertures are periodically spaced along the mainwaveguide.

The filter 20 illustrated in FIGURES 2 and 3 follows the conceptdescribed with respect to the filter of FIGURE 1 and represents apreferred embodiment. The filter isconstructed of the same two basicelements as the filter of FIGURE 1; that is, the filter includes awaveguide segment 21 through which electromagnetic waves of the sys-6111. are propagated and a lossy or absorptive array 22 coupled to thewaveguide to remove and absorb harmonics of the fundamental and unwantedmodes propagated in the main guide 21.

The periodic absorptive structure22 illustrated consists of an array ofparallel waveguides 23 of rectangular cross section each of which isperpendicular to the main waveguide 21. That is, the ion imdinataa f. chwav waveguide section 15 in the filter of FIGURE 1, the cross sectionaldimensions of each of the waveguide sections 23 which make up the array22 are selected so that electromagnetic waves in the band of frequenciesto be eliminated from the system are readily propagated and energy inthe band of frequencies of interest are not propagated. Thus, thewaveguide sections 23 are said to be beyond cut off for the fundamentalfrequencies.

The absorptive array 22 is a composite structure which is essentiallydefined by two rows or series of the individual waveguide sections 23.In each series the individual sections 23 are parallel and arrangedbroad wall to broad wall so that a plane passing through thelongitudinal axis or each of the sections 23 in the particular seriesbisects the broad walls. The. two rows or series of waveguide sections23 are arranged side by side.

From an inspection of FIGURES 2 and 3, it is seen that the absorptivearray 22 is not actually constructed of a plurality of separatewaveguides but is formed by stacking a plurality of rectangularconductive plates 24 and conductive rods 25 of rectangular cross sectionwith a pair of the rods 25 between opposite edges of each pair of theplates 24 to define a plurality of broad waveguides. The two series ofside-by-side rows of waveguide sections 23 are formed by dividing thestructure down its length with a planar conductive dividing vane 26. Thestructure is formed into a unit by brazing all of these parts together.

In order substantially to eliminate the energy coupled to the individualwaveguide sections 23 from the system, a broad band absorptivetermination 27 is provided in each waveguide section. In the absorptivearray 22, illustrated, the termination in each individual waveguidesection 23 comprises a planar resistive card 27 positioned in the endwhich is farthest away from the coupling between the main waveguide andthe segment, in such a manner that it extends between and bisects thebroad walls of the section 23. The lower end of each of the resistivecards 27 is tapered to provide a better impedance match and terminationwithin the waveguide section. The resistive card terminations 27 in eachseriesor row of waveguide sections 23 are reversed in successivesections in such a manner that the arrangement of resistive cards 27 onany broad wall between two waveguide sections 23 is allochiral. Thus, ina sectional side view through the array 22 showing the resistive cards27 (see FIGURE 2), the cards 27 give the appearance of a series ofarrows pointing down toward the main waveguide 21 at alternate broadwalls of the waveguide sections 23.

In the arrangement illustrated, the lossy array 22 is coupled to themain waveguide through apertures defined by the open ends of thewaveguide sections in the array. This is accomplished by removing orcutting away a portion of the top or upper broad wall 28 of thewaveguide segment 21 and fitting the array 22 into the aperture thusformed in such a manner that open ends of the waveguide sections 23 areexposed to the interior of the main guide 21. Since the cross sectionaldimension of the waveguide sections 23 or individual cells in theabsorptive array is such that individual cells 23 do not propagateelectromagnetic waves of the fundamental frequency, the apertures whichcouple the harmonics to the lossy array simply represent a reactiveloading to the main guide 21 as far as the fundamental is concernedwithout otherwise interfering with propagation of the fundamental. Onthe other hand, the apertures freely couple harmonic energy to theabsorptive array 22. y

In order properly to match the impedance of the main waveguide 21 to theindividual cells or waveguide sections 23 in the absorptive array 22 forthe undesired harmonics, it has been found desirable to reduce thedistance between upper and lower broad walls 28 and 30 respectively,i.e.,. the height of the narrow walls 31 and 32 of the main:waveguide'segment in a. taper.. This produces better. cou-- plingbetween the main waveguide segment 21 and the absorptive array 22 sinceit increases the concentration of electric fields in the coupling regionand reduces the main guide impedance to a value more nearly equal to theimpedance of the smaller waveguide sections 23 in the lossy array 22.Thus, the resultant shape of the main waveguide segment is defined by aplanar upper broad wall 28 and planar parallel side walls 31 and 32 anda curviplanar lower broad wall 30 which is at its greatest distance fromthe upper broad Wall at opposite ends of the segment 21 and curvestoward the upper broad wall in the center of the segment 21. In theparticular main waveguide segment 21 illustrated, the curve of the lowerbroad wall defines a double cosine taper.

The particular filter of FIGURES 2 and 3 was designed specifically topass energy having frequencies between two and four kilomegaeycles andabsorb harmonic energy. In other words, the main waveguide segment 21 isselected to have a large enough cross sectional dimension freely topropagate energy in the pass band while the waveguide sections 23 in thearray 22 exhibit distinct cut off properties at the upper edge of thepass hand. To accomplish this, the array 22 is constructed of waveguidesections which have internal dimensions of about /2" X 1%". Each of therows of sections in the array 22 is eight inches long and containstwenty-six sections. The curve of FIGURE 4 shows the transmissioncharacteristics of this filter.

In the graph of FIGURE 4, the insertion loss in decibels and therefiected power in terms of the Voltage Standing Wave Radio (VSWR) areplotted along the axis of the ordinates and the frequency in kmc. isplotted along the axis of the abscissas. The insertion loss in decibelsis a measure of the ratio of the power delivered by a source to amatched load through the filter (designated P to the power delivered bythe source to the matched load without the filter in the system(designated P and the value of the insertion loss in decibels isobtained by the following formula:

The Voltage Standing Wave Radio represents the ratio of the maximumvoltage (V measured along the guide 20, to the minimum voltage measuredtherealong, i.e., V /V The VSWR is unity when there are no refiectionsin the guide, i.e., there is no standing wave in the waveguide andconsequently the voltage is constant over the length or" the guide. Thisrepresents the ideal condition.

As is seen from the Voltage Standing Wave Radio (VSWR) curve for thefundamental, i.e., in the pass band, the value is well below 1.5 overthe entire band if a slight discontinuity corresponding to a periodicVSWR variation is tuned out. This is relatively easily accomplished. Theinsertion loss in the pass band is less than 0.2 db over almost theentire pass band and is less than 1 db over the full pass band. Thesefigures indicate that the particular filter is extremely good over theentire pass band. A VSWR of 1.2 is considered by systems people to beexcellent whereas tube people strive for a VSWR on the order of 2.5 db.Generally, any insertion loss in the pass band which is less than 1 dbis considered to be excellent, whereas .2 db or less as found above isbeyond expectations for most high power applications.

In order to measure the transmission characteristics of the filter athigher frequencies, i.e., in the stop band, it is necessary to launchmore than one mode in the waveguide; that is, the transverse electricmodes, TE and TE Inspection of FIGURE 4 illustrate-s that the VSWR ofthe TE mode is below 1.05 over almost the entire pass band. Theinterpretation of the VSWR curve for the TB mode requires anunderstanding that the mode launching hybrid utilized to launch thismode did not have a low VSWR except in the frequency range of from 5.7to 6.4 kmc. Outside of this region, then, the measure- Insertion L0ss=10log 6 ments illustrated practically describe the characteristic of thehybrid which is highly reflective. However, it is seen that the VSWR forthe TW mode is below 1.25 within the range of validity.

The insertion loss of the filter in the stop band for the TE and the TEmodes illustrates the excellent properties which can be obtained. Forthe TE mode, the insertion loss remains fairly constant, at leastthrough the frequency range of measurements, at a value above 40 dbwhile the insertion loss of the TE mode remains above 40 db for almostthe entire frequency range of measurements. In general, an insertionloss for the unwanted modes in the stop band is considered excellent ifit remains above 30 db.

It is recognized that at least three other modes could appear due tosecond harmonics in the system. However, the transmissioncharacteristics illustrated and described here are considered to be themost important and the conclusions reached using these measurements arecon sidered to be entirely valid since the TE and the TE modes are the:most likely to give trouble. In addition, it is extremely difficult tolaunch and measure the other possible modes.

The absorptive array 35 illustrated in FIGURE 5 is another periodicstructure which may be utilized with the waveguide segment 21 of FIGURES2 and 3 in lieu of the absorptive array 22 illustrated. This array 35 isalso a composite structure of waveguide sections 36 having crosssectional dimensions which are proportioned to render the sectionsbeyond cut off for the fundamental electromag netic wave energy in themain waveguide. However, the

waveguide sections 36 are arranged in what may be termed.

two series. A plurality of pairs of individual waveguide sections 36with the individual sections or cells of each pair positioned side byside with adjacent narrow walls form one such series. The other seriesof individual waveguide sections is made up of a plurality of singlesections 36. The two series of waveguide sections are intercalated insuch a manner that the common narrow walls of the series formed by thepairs of sections are coplanar and the plane so defined contains thelongitudinal axes of the individual single sections in the second seriesof sections. In this manner, an electromagnetic wave being propagateddown the main guide first encounters the coupling apertures to a pair ofside by side waveguide sections 36, i.e., two sections divided by acentral vane, and then the coupling aperture to a single waveguide 36which is also perpendicular to the main guide segment and is centrallylocated with respect to the broad wall of the main guide. Theelectromagnetic wave successively encounters such arrangements down thelength of the array 35. Thus, the array is very effective for couplingto both symmetrical and unsymmetrical modes in the main waveguidesegment.

The absorptive array 35 of FIGURE 5 is also a stacked compositestructure. The structure includes a plurality of rectangular planarconductive plates 37 similar to the plates 24 utilized for the array 22of FIGURES 2 and 3, a first series of conductive spacer bars havingrectangular cross sections of the same dimensions as the bars 25 of thearray 22 and a second series of conductive spacer bars 39 havingrectangular cross sections and the same thickness as the first series ofspacer bars 38 but having a greater lateral dimension. The array isfabricated by stacking the conductive plates 37 with a pair of spacerbars between the outer edges of each pair of plates and using a pair ofbars from alternate series between alternate plates. In this manner, theplates 37 form the broad walls of all of the waveguide sections 36 inthe array 35 but alternate guides so formed have a broad wall which istwice as broad as the remaining guides. These broad guides are thenbisected by conductive divider vanes 40 to form side by side pairs ofwaveguide sections 36 having the same dimensions as the interveningsingle sections.

The broad band absorptive loads for the individual waveguide sections 36of FIGURE 5 are not illustrated 7 although they are essential. They maybe resistive cards of the type described in connection with FIGURES 2and 3 and they may be similarly positioned in the waveguide sections 36.

The reason for using different arrangements of absorptive waveguidesections 36 in absorptive arrays, is to provide better coupling betweenthe main guide and the absorptive array for those modes which it is mostdesirable to absorb in the array. For example, by definition, thetransverse electn'c mode TE means that the electric field in the guideis perpendicular to the sides of the guide and has no component alongthe length of the axis of the guide, and the subscript (20) means thatthere are two full half patterns of the electric field encountered whenmoving from one side or narrow wall of the guide to the opposite narrowwall of the guide while there are no half patterns encountered inpassing across the other side of the cross section, i.e., between broadwalls. It would be expected that this mode would couple well to anabsorptive array with a central dividing vane, since the dividing vanepresents a minimum of perturbation due to the fact that it occurs at thepoint in the main guide 21 where there is essentially no electric field;that is, at the point where the electric field makes its transition fromone direction to the opposite direction. It is to be expected that sucha mode would couple better to the absorber cells than modes wherein theelectric field is a maximum at the dividing vane. The array 22 ofFIGURES 2 and 3 and the series of side by side sections of the array ofFIGURE are examples of arrangements with good coupling to the TE mode.

Using similar reasoning, it would be assumed that the TE mode and otherassymetrical modes would couple best, i.e., with least perturbation, toan absorptive array with coupling apertures at the center of the broadwall of the guide. This has proved to be the case. It follows, then,that one effective type of absorptive array is an array whereinsuccessive portions of the array are coupled to the main guide bycoupling apertures in different positions but still periodically alongthe length of the main guide. The array of FIGURE 5 is such a device.

In arrangements just described, energy in themain guide 21 essentiallyis deflected when entering the absorber in the direction perpendicularto its previous path of propagation. Such a deflection is not. arequirement of the absorptive type filter. For example, FIGURE 6illustrates a filter 42 of the absorptive type in which the energy inthe main giude 43 and in the absorptive arrays44 propagate in thedirections which are parallel. The main waveguide segment 43 to beinserted in the waveguide system and propagate the electromagnetic wavesin the system is a common rectangular guide in the particular filterillustrated. Identical absorptive arrays 44 are positioned on each ofthe broad walls 45 of the rectangular guide 43. Each array is comprisedof three side-by-side rectangular. waveguide sections 46 having coplanarbroad walls 47 which extend along the length of the main guide structureparallel with the broad walls 45 of the main Waveguide 43. Eachabsorptivearray 44 constitutes a periodic structure by virtue of slotsor coupling apertures 48 which are periodically spaced down the lengthof the guides.

Like the dimensions of coupling apertures for absorptive arrayspreviously discussed, the dimensions of the coupling apertures 48 aresuch that they represent inductive loading to electromagnetic waveenergy of the fundamental frequencies in the main waveguide segment 43but serve to couple the desired harmonic frequencies from the main guideto the auxiliary absorptive guides 46. Absorption of the harmonicfrequencies is accomplished in each of the waveguide sections 46 bypositioning a strip of absorptive material 50 such as a ceramicimpregnated with resistive material, on the inside of the broad wall 47of the guide opposite the main waveguide. The lossy strips 50 are placedon the wall of the absorptive waveguide sections 46 opposite thecoupling apertures 48 so that they perform their function withoutinterfering with wave energy of the fundamental frequency in the mainwaveguide segment.

Structurally, each of the lossy arrays 44. illustrated is formed bybrazing three waveguides 46 together with coplanar broad walls 47 andcutting parallel coupling slots 43 all the way across one set of broadwalls of the resultant array at intervals down the length. A section ofthe broad walls of the main guide 43 is removed and each absorptivearray is positioned in the aperture thus formed to expose the couplingapertures 48 to the interior of the main guide segment 43.

Each of the absorptive arrays 44 on the filter illustrated in FIGURE 6effectively contains two dividing vanes, i.e., forms three side by sidewaveguide segments and hence represents a structure which isparticularly effective for absorbing the transverse electric modes TEand TE for the reasons given in connection with the discussionconcerning the lossy arrays 22 and 35 of the filter illustrated inFIGURES 2 and 3, and FIGURE 5. That is, the dividing vanes between thewaveguide sections 46 of the lossy arrays 44 are positioned to provide aminimum perturbation of the electric fields of such modes and hence amaximum coupling between for these modes.

FIGURE 7 illustrates an absorptive periodic structure 44 of the typejust described in connection with FIGURE.

6 utilized with a double cosine tapered waveguide segment 21 of the typedescribed in connection with FIGURES 2 and 3. Corresponding elements ofthe various figures are given the same reference numerals to simplifythe description and drawings. This arrangement has the advantage ofproviding better coupling to the waveguide sections 46 in the lossyarray 44 as described in connection with the double cosine taperedWaveguide segment 21 of FIGURE 2. The .device is constructed bypositioning one of the absorptive arrays 44, illustrated and describedin connection with FIGURE 6, in the aperture provided by the removedportion of the upper wall 28 of the waveguide segment 21.

Actual tests performed using this filter as illustrated indicate thatthe over all transmission characteristic of the fundamental as well asthe third harmonic is excellent in terms of the VSWR. The insertion lossfor the fundamental is'better than 0.1 db. The tests are conclusive inshowing that the fundamental in the waveguide segment 21 is not affectedby the lossy strips if the proper dimensions of the coupling apertures48 and the Waveguide sections 46 are selected and the absorber strips 50are properly positioned. Aside from empirical results, the best guide toplacing the lossy strips 50 where they do not affect the fundamental, isto place them in the waveguide sections 46 of the absorptive array 44 insuch a manner that they are as far removed as practicable from couplingapertures 48. t

The filter of FIGURE 8 is another frequency selective absorptive filterutilizing the same principles as the filters described in connectionwith FIGURES 6 and 7 but having absorptive arrays of several differentconstructions to provide absorption of energy of selected modes inaccordance with discussion given in connection with the placement ofdividing vanes in the absorptive arrays of FIGURES 2, 3 and 5.

The main waveguide segment 52 of the filter is a rectangular waveguide.The absorptive array 44 which is utilized in connection with the lowerbroad wall 53 of the filter, is-of the same construction as theabsorptive arrays utilized in connection with the filters of FIGURES 6and 7 except that the coupling apertures 54 of this array are This lossyarray 55 is constructed of a pair of waveguide sections 57 ofrectangular cross section placed side by side with their longitudinalaxes parallel and parallel to the longitudinal axis of the mainwaveguide segment 52. The dimensions of the broad wall of the twowaveguide sections 57 are such that the two guides placed side by sideare coextensive with the broad walls of the main guide segment 52.Coupling apertures 58 for each of the sections 57 are provided andconstitute a succession of relatively narrow slots which are spacedperiodically down the length of the lower broad wall of the waveguidesections 57. Lossy strips 66 are placed integrally adjacent the upperbroad walls of the sections 57 to absorb electromagnetic energy therein.

The filter is also provided with two absorptive arrays 61 on oppositesides of the main waveguide segment 52, each of which constitutes asingle waveguide section 62 of rectangular cross section having itsbroad wall dimension coincident with the narrow wall dimension of themain waveguide segment 52 and coupling apertures 63 defining relativelynarrow slots which extend across the broad wall of the waveguide section62 periodically disposed down the length of the waveguide segment. Againa strip of absorbent material 64 is placed along and coincident with thebroad wall of each of the absorptive arrays 61 which is opposite thecoupling apertures 63. In practice, the filter is constructed by puttingthe absorptive arrays 44, 45 and 61 together as described and thenbrazing the arrays so that they define the central portion of thewaveguide segment 52. Pieces of rectangular waveguide 65 having thecross section of the main segment 52 thus defined are brazed to oppositeends of the lossy arrays to provide the desired filter length.

The filter of FIGURE 9 utilizes two absorptive arrays 70 which have sideby side waveguide sections 71 similar to the upper absorptive arrays 55described with respect to FIGURE 8. Each of the two waveguide arrays isarranged on an opposite broad wall of the main waveguide. The maindifierence between the absorptive arrays utilized with the filter ofFIGURE 9 is that the coupling apertures 72 provided in each waveguidesection '71 are of a slightly different configuration from those of FIG-URE 9 and the energy absorption is provided in a little different way.In the embodiment of FIGURE 9, the coupling apertures 72 are almostelliptical. However, they are again periodically spaced along the lengthof each section 71. The lossy material in each waveguide section 71consists of blocks of carbonized ceramic '73 which fill the entire crosssection of each waveguide cell 71 between successive coupling apertures72.

FIGURES and 11 and FIGURE 12 illustrate absorptive filters of ridgedWaveguide configuration. Since the main elements of these filterscorrespond exactly, they are given identical reference numerals tosimplify the description and drawings. The main waveguide segments 75 ofthe filters are of rectangular cross section and have identical hollowrectangular ridges 76 on opposite broad walls 77 wherein the ridgesessentially form rectangular waveguides with their broad walls 78parallel to the broad walls 77 of the main waveguide 75. Each of theridges 76 constitutes a periodic absorptive structure Which is formed inthe embodiment of FIGURES 10 and 11 by providing coupling apertures 79which constitute a pair of slots extending across the top of each ridgerelatively close together and an absorptive resistive card 80 and thenanother pair of coupling slots 79 and so on over the length of the ridgeforming waveguide sections 76. The absorptive cards 86 are substantiallyplanar and extend all the way across the broad dimension of the guidesection 76 in a plane substantially half way between the broad walls ofthe waveguide. The cards are tapered at both ends, i.e., the endsnearest the coupling apertures, in such a manner that the tapered endshave the general appearance of arrows directed toward the nearest set ofcoupling apertures.

In the filter of FIGURE 12, each of the ridges 76 in the waveguidesegment constitutes an absorptive periodic structure which is formed bymilling a series of cou pling apertures, slots 81, across the broad wallof the ridges nearest the center of the guide segment 75 at periodicintervals along the length of the ridge. Absorption of electromagneticwaves in each of the ridge guide sections 76 is provided by a flat stripof carbonized ceramic 82 positioned on the broad Wall of the absorptiveperiodic structure opposite the coupling apertures 81.

FIGURES 13 and 14- and FIGURES 15 and 16 illustrate two embodiments ofabsorptive filters which use cylindrical waveguide segments 85. Theabsorptive structure in each of these cylindrical filters constitutes ahollow cylindrical waveguide section 86 coaxially disposed within themain cylindrical waveguide segment 85. In both embodiments, sets of fourcoupling apertures 87 of essentially circular configuration areperiodically spaced down the length of the absorptive waveguide section86. In each case, the four coupling apertures 87 in each set are spacedequidistant around the periphery of the absorptive waveguide section 86.The only difiterence between the filter arrangement illustrated inFIGURES 13 and 14 and that in FIGURES 15 and 16 is in the configurationof the absorptive elements. The absorbers illustrated in the filter ofFIGURES 13 and 14 are planar resistive cards 88 having substantially thesame configuration as the resistive cards $0 illustrated in the ridgedfilter of FIG- URES 10 and 11. That is, they extend entirely across theinternal diameter of the waveguide section 86 and are tapered toward thecoupling apertures 87 on opposite ends. In the embodiment of FIGURES l5and 16, solid blocks 90 of absorptive material such as a carbonizedceramic fill the entire Waveguide section 86 between the couplingapertures 87 along the length of the waveguide section. As with theother absorptive sections, the dimensions of the centrally locatedabsorptive section 86 in both filters is such that as to precludepropagation of energy of the fundamental in the centrally locatedsection.

Another type of absorptive array in which the energy in the main guidesegment essentially is deflected in a direction perpendicular to itsprevious path of propagation when entering the absorber, is illustratedin FIG- URE 17. In this arrangement, the main waveguide segment 91 is ofrectangular cross section, the absorptive array 92 is made of aplurality of waveguide sections 93 of rectangular cross sectionpositioned side by side with their longitudinal axes parallel to eachother and perpendicular to the direction of propagation in the mainwaveguide segment 91. However, unlike the absorptive arrays of FIGURES 2and 5, the Waveguide segments 91 are all positioned with theirlongitudinal axes in a plane parallel with the upper broad wall 94 ofthe main waveguide segment 91. The waveguide sections 93 are all coupledto the main waveguide 91 by removing a section of the top broad wall ofthe Waveguide sections 93 which are adjacent the main waveguide 91.

In the embodiment illustrated, the coupling apertures 95 aresymmetrically arranged on opposite sides of the center line of the broadwall of the main waveguide 91 and each end of each waveguide section 93contains a resistive card 96 of planar construction positioned along thelongitudinal axis of the section 93 in a vertical plane to define acentrally located dividing vane. In order to assure a non-reflectivematch in each waveguide section 93, the end of the resistive cards 96which are nearest the coupling apertures 95 are sheared off at an angle.Once again, the cross section of the guide sections 93 in the absorptivearray 92 is selected to prevent propagation of the fundamentalelectromagnetic energy from the main waveguide segment 91 and thecoupling apertures 95 are selected to couple the desired frequencies andmodes from the waveguide segment 91 to the array 92.

Thus, the objects of the present invention are carried out by providinga main guide propagating fundamental and harmonic power and auxiliaryguides or absorbers with coupling from the main guide to the absorbersthrough apertures designed to match the fields of waves propagating fromthe main guide into the absorber wherein the energy entering theabsorber is dissipated in low reflective wide band loads and thefrequency selectivity of the device results from the fact that theabsorbers are chosen to exhibit distinct cut off properties at certainfrequencies denoting the upper end of the pass band in the absorptivefilters thus obtained. Below the cut off frequency of the absorber, thecoupling apertures merely represent the reactive loading of the mainwaveguide segment, while at frequencies above cut off, energy will enterthe absorber and be dissipated in its load.

While particular embodiments of the invention have been illustrated, itwill, of course, be understood that the invention is not limited tothese embodiments, since many modifications may be made. It iscontemplated that the appended claims will cover any such modificationsas fall within the true spirit and scope of the invention.

What is claimed is:

1. A waveguide filter for transmitting first electromagnetic waveshaving predetermined fundamental frequency and for absorbing secondelectromagnetic waves of higher frequency, comprising: an elongatedwaveguide segment for transmitting therethrough said firstelectromagnetic wave; a wave energy absorbing means for eliminatingsubstantially all wave energyof all frequencies above said predeterminedfrequency from said Waveguide segment without substantial attenuation ofthe wave energy of frequencies below said predetermined frequency, saidmeans comprising a plurality of waveguide sections having crosssectionaldimensions preventing propagation ofsaid first electromagnetic waves andallowing propagation therein of said second electromagnetic waves, aplurality of apertures coupling said structure to said segment, eachaperture directly coupling said segment to one of said sections forcoupling said second electromagnetic Waves from said segment to saidwaveguide sections said apertures being spaced apart along the length ofsaid segment at intervals which are less than one-quarter wavelength ofsaid first waves; and wave absorbent material in said waveguide sectionsto dissipate said second electromagnetic waves, said absorbent materialbeing disposed sufiiciently remote from said apertures to avoidsubstantial interception of said first electromagnetic waves.

2. An electromagnetic wave filter for transmitting wave energy offrequencies within a predetermined pass frequency band and for absorbingwave energy of frequencies above said pass frequency band, comprising:an elongated main waveguide for receiving electromagnetic wave energy,said main waveguide having cross-section dimensions to propagate wavesof. frequencies within said pass frequency band; and wave energyabsorbing means for eliminating substantially all wave energy offrequencies above said pass frequency band from said main waveguidewithout substantial attenuation of the pass frequency band wave energyincluding a plurality of secondary waveguide segments coupled to saidmain waveguide, each of said secondary waveguide segments havingcross-section dimensions such that said segments are above cutoff forwaves of frequencies within said pass frequency band, each of saidsecondary waveguide segments being directly coupled to said mainWaveguide by at least one coupling aperture, the area of each couplingaperture being substantially equal to the cross-section area ofitsassociated secondary waveguide segment; and load means disposed todirectly receive wave energy propagated by said secondary waveguidesegments, said load means being positioned a sufficient distance fromsaid coupling apertures to avoid substantial disturbance of waves offrequencies. within said pass frequency band.

3.. An electromagnetic wave filter for transmitting wave energy offrequencies within a predetermined pass frequency band and for absorbingwave energy of frequencies above said pass frequency band, comprising:an elongated main waveguide for receiving electromagnetic wave energy,said main waveguide having cross-section dimensions to propagate wavesof frequencies within said pass frequency band; and wave energyabsorbing means for eliminating substantially all wave energy offrequencies above said pass frequency band from said main waveguidewithout substantial attenuation of the pass frequency band wave energyincluding a plurality of secondary waveguide segments coupled to saidmain waveguide, each of said secondary waveguide segments havingcross-section dimensions such that said segments are above cutoff forwaves of frequencies within said pass frequency band, each of saidsecondary waveguide segments being directly coupled to said mainwaveguide by at least one coupling aperture, the areavof each couplingaperture being substantially equal to the cross-section area of itsassociated secondary waveguide segment, the spacing between saidapertures being less than one-quarter wavelength at the upper frequencylimit of said pass frequency band; and load means disposed to directlyreceive wave energy propagated by said secondary waveguide segments,said load means being positioned a sufficient distance from saidcoupling apertures to avoid substantial disturbance of waves offrequencies within said pass frequency band.

4. An electromagnetic wave filter for transmitting wave energy offrequencies within a predetermined pass frequency band and for absorbingwave energy of frequencies above said pass frequency band, comprising: amain waveguide segment for receiving electromagnetic wave energy, saidmain waveguide segment being designed to propagate waves of frequencieswithin said pass frequency band; a wave energy absorptive means foreliminating substantially all wave energy of all frequencies above saidpredetermined frequency from said waveguide segment without substantialattenuation of the wave energy of frequencies below said predeterminedfrequency, said means comprising an array of waveguide sections arrangedwith parallel axes including at least two series of such sections, eachof said waveguide sections having cross section dimensions which preventpropagation therein of wave energy of frequencies within said passfrequency band and which allow propagation therein of wave energy offrequencies above the upper frequency limit of said passfrequency band,each of said waveguide sections being coupled to said main waveguidesegment by at least one coupling aperture; and electromagnetic energyabsorbent material positioned to directly receive the wave energypropagated by said waveguide sections.

5. An electromagnetic wave filter for transmitting wave energy offrequencies within a predetermined pass frequency band and for absorbingwave energy of frequencies above said pass frequency band, comprising: amain waveguidesegment for receivingelectromagnetic wave energy, saidmain waveguide segment being designed to propagate waves of frequencieswithin said pass frequency band; and a wave energy absorptive means foreliminating substantially all wave energy of all frequencies above saidpredetermined frequency from said waveguide segment without substantialattenuation of the wave energy of frequencies below said predeterminedfrequency, said means comprising an array of waveguide sectionsincluding at least two series of such sections, said series of saidsections being arranged in sets of at least two adjacent series of saidsections, each of said waveguide sections having cross sectiondimensions which prevent propagation therein of wave energy of.frequencies within said pass frequency band and which allow propagationtherein of wave energy of frequencies above the, upper frequency limitof said pass frequency band, each of said waveguide sections beingcoupled to said main waveguide segment by at least one couplingaperture.

6. An electromagnetic wave filter for transmitting wave energy offrequencies within a predetermined pass frequency band and for absorbingwave energy of frequencies above said pass frequency band, comprising:an elongated main waveguide for receiving electromagnetic wave energy,said main Waveguide being designed to propagate waves of frequencieswithin said pass frequency band; and wave energy absorbing means foreliminating substantially all wave energy of frequencies above said passfrequency band from said main waveguide Without substantial attenuationof the pass frequency band energy including a plurality of secondarywaveguide segments, each designed to propagate wave energy offrequencies above said pass frequency band and to prevent propagationtherein of wave energy of frequencies within said pass frequency band;and a plurality of coupling apertures, each of said secondary waveguidesegments being directly coupled to said main waveguide by at least oneof said apertures, the spacing of said apertures along the length ofsaid main waveguide being less than one-quarter Wavelength at thefrequency of the upper limit of said pass frequency band.

7. An electromagnetic wave filter for transmitting wave energy offrequencies within a predetermined pass frequency band and for absorbingwave energy of frequencies above said pass frequency band, comprising:an elongated main waveguide for receiving electromagnetic wave energy,said main waveguide being designed to propagate waves of frequencieswithin said pass frequency band; and wave energy absorbing means foreliminating substantially all wave energy of frequencies above said passfrequency band from said main waveguide without substantial attenuationof the pass frequency band energy including a plurality of secondarywaveguide segments, each designed to propagate wave energy offrequencies above said pass frequency band and to prevent propagationtherein of wave energy of frequencies within said pass frequency band; aplurality of coupling apertures, each of said secondary waveguidesegments being directly coupled to said main waveguide by at least oneof said apertures, the spacing of said apertures along the length ofsaid main waveguide being less than one-quarter Wavelength at thefrequency of the upper limit of said pass frequency band; and waveenergy absorbent material disposed to directly receive wave energypropagated by said secondary waveguide segments.

8. An electromagnetic wave filter for transmitting with low attenuationwave energy of frequencies within a predetermined pass frequency bandand for absorbing 'wave energy of frequencies above said pass frequencyband, comprising: an elongated main waveguide segment for receivingelectromagnetic wave energy, said main waveguide segment being designedto propagate waves of frequencies within said pass frequency band; andwave energy absorbing means for eliminating substantially all waveenergy of frequencies above said pass frequency band from said mainwaveguide without substantial attenuation of the pass frequency bandenergy including a p1u rality of coupling apertures in said mainwaveguide segment; a plurality of absorber waveguide segments, eachdirectly coupled to at least one of said apertures, and each of saidabsorber waveguide segments having crosssection dimensions which preventpropagation therein of wave energy of frequencies within said passfrequency band and which allow propagation therein of wave energy offrequencies above the upper frequency limit of said pass frequency band;and a low-reflective wideband load disposed within each of said absorberwaveguide segments for dissipating wave energy propagated by saidabsorber waveguide segments.

9. An electromagnetic wave filter for transmitting wave energy offrequencies within a predetermined pass frequency band and for absorbingwave energy of frequencies above said pass frequency band, comprising: arectangular main Waveguide for receiving electromagnetic energy ofsaidmain waveguide having broad and narrow wall dimensions to propagatewaves having frequencies within said pass frequency band; and waveenergy absorbing means for eliminating substantially all wave energy offrequencies above said pass frequency band from said main waveguidewithout substantial attenuation of the pass frequency band energyincluding at least two secondary waveguide arrays each coupled to arespective wall of said main waveguide, each secondary waveguide arraycomprising at least two adjacent series of secondary waveguides, each ofsaid secondary waveguides having cross-section dimensions above cutofffor wave energy of frequencies Within said pass frequency band; andmeans coupling each of said secondary waveguides to said main Waveguide.

10. An electromagnetic Wave filter for transmitting wave energy offrequencies within a predetermined pass frequency band and for absorbingwave energy of frequencies above said pass frequency band, comprising:an elongated main waveguide for receiving electromagnetic Wave energy;said main waveguide having cross-section dimensions to propagate wavesof frequencies within said pass frequency band; and wave energyabsorbing means for eliminating substantially all wave energy offrequencies above said pass frequency band from said main waveguidewithout substantial attenuation of the pass frequency band energyincluding a plurality of secondary waveguides connected to said mainwaveguide, each of said secondary waveguides being coupled to said mainwaveguide by a respective coupling aperture, the cross-sectiondimensions of said secondary waveguides and the size, shape andorientation of said coupling apertures being selected to couple waveenergy of frequencies above said pass frequency band from said mainwaveguide into said secondary waveguides without substantial disturbanceof wave energy of frequencies within said pass frequency band; and alow-reflective, wide-band load disposed to directly receive wave energypropagated by said secondary waveguides.

11. An electromagnetic wave filter for transmitting wave energy offrequencies within a predetermined pass frequency band and for absorbingwave energy of frequencies above said pass frequency band, comprising: arectangular main waveguide for receiving electromagnetic energy, saidmain waveguide having broad and narrow wall dimensions to propagatewaves having frequencies within said pass frequency band; and waveenergy absorbing means for eliminating substantially all wave energy offrequencies above said pass frequency band from said main waveguidewithout stubstantial attenuation of the pass frequency band energyincluding a plurality of planar conductive plates conductively joined toat least one wall of said main waveguide and positioned normal thereto,said plates forming at least two adjacent series of waveguide sectionshaving cross-section dimensions such that said sections are beyondcutoff for waves having frequencies within said pass frequency band; aplurality of coupling apertures in said one wall of said main waveguide,each of said coupling apertures providing direct coupling between saidmain waveguide and a respective one of said waveguide sections for waveenergy of frequencies above said pass frequency band; and a loW-reflectivity wideband load disposed to directly receive wave energypropagated by said waveguide sections.

12. An abosorptive filter for removing electromagnetic wave energy froma waveguide system wherein desired wave energy of fundamental frequencyand undesired Wave energy of higher frequency is propagated, thecombination of: a waveguide segment to be inserted in the waveguidesystem such that electromagnetic waves therein must pass through saidwaveguide segment; an absorptive means for eliminating substantially allwave energy of all frequencies above said predetermined frequency fromsaid wave guide segment without substantial attenuation of the waveenergy of frequencies below said predetermined frequency, said meanscomprising an array of waveguide sections having cross sectionaldimensions which preclude propagation of wave energy of said fundamentalfrequency therein but which allow propagation therein of said undesiredwave energy, said sec- :said segment whereby said coupling apertures arespaced along the length of said segment at intervals which are less thanone-quarter wavelength at said fundamental frequency; andelectromagnetic energy absorbent material positioned in each of saidwaveguide sections to absorb electromagnetic wave energy therein.

13. An electromagnetic wave filter for transmitting wave energy offrequencies within a predetermined pass frequency band and for absorbingwave energy of frequencies above said pass frequency band, comprising: arectangular main waveguide for receiving electromagnetic energy, saidmain waveguide having broad and narrow wall dimensions to propagatewaves having frequencies within said pass frequency band; and waveenergy absorbing means for eliminating substantially all wave energy offrequencies above said pass frequency band from said main waveguidewithout substantial attenuation of the pass frequency band wave energyincluding a plurality of planar'conductive plates conductively joined toat least one wall of said main waveguide and positioned normal thereto,said plates forming a series of waveguide sections having cross-sectiondimensions such that said sections are beyond cutoff for waves havingfrequencies within said pass frequency band; a plurality of couplingapertures in said one wall of said main waveguide, each of said couplingapertures providing direct coupling between said main waveguide and arespective one of said waveguide sections for wave energy of frequencies above said pass frequency band; and a low-reflectivity wideband load disposed to directly receive wave 'energy propagated by saidwaveguide sections.

14. An electromagnetic wave filter for transmitting wave energy offrequencies within a predetermined pass frequency band and for absorbingwave energy of frequencies above said pass frequency band, comprising: arectangular main waveguide for receiving electromagnetic energy, saidmain waveguide having broad and narrow wall dimensions to propagatewaves having frequencies Within said pass frequency band; and waveenergy absorbing means for eliminating substantially all wave energy offrequencies above said pass frequency band from main waveguide withoutsubstantial attenuation of the pass frequency band wave energy includingat least one secondary waveguide array coupled to at least one wall ofsaid main waveguide, each secondary waveguide array comprising aplurality of elongated secondary waveguide channels, each secondarywaveguide channel having an input end and a load end, the input ends ofsaid secondary waveguide channels being positioned adjacent said onewall of said main waveguide, each of said secondary waveguide channelshaving cross-section dimensions such that said channels are above cutotffor wave energy of frequencies within said pass frequency band, saidsecondary waveguide channels being positioned as closely together as ispractically possible along said one wall of :said main waveguide;aplurality of coupling openings, each opening being formed in said oneWall of said main waveguide in a position adjacent the input end of are- :spective one of said secondary waveguide channels, each of saidopenings having as large a size as is practically possible consistentwith the cross-section dimensions of its respective secondary wave-guidechannel; and load means disposed at said load end of each said secondarywaveguide channels for directly receiving and absorbing wave energypropagated therein, said load means being positioned at a distance fromsaid one wall of said main waveguide sutficient to avoid substantialabsorption of wave energy of frequencies within said pass frequencyband.

15. The filter of claim 14 wherein adjacent walls of adjacent ones ofsaid secondary waveguide channels are formed by a cQ D UQ-n PQnductivemember.

16. A frequency selective absorptive filter for removing electromagneticwave energy of predetermined frequencies and modes from a waveguidesystem wherein electromagnetic wave energy of desired fundamentalfrequency and undesired electromagnetic wave energy of higher frequencyis propagated including the combination of a waveguide segment to beinserted in the waveguide system such that electromagnetic waves thereinmust pass through said waveguide segment; an absorptive means foreliminating substantially all wave energy of all frequencies above saidpredetermined frequency.

from said waveguide segment without substantial attenuation of the waveenergy of frequencies below said predetermined frequency, said meanscomprising an array of rectangular waveguide sections including at leasttwo series of such sections, the wave guides in each of said seriesbeing arranged with adjacent broad walls and each series positionedadjacent the other series whereby narrow side walls of one series areadjacent narrow walls of said other series, each of said waveguidesections having cross sectional dimensions such that said sections arebeyond cutoff for the fundamental electromagnetic wave energy but whichallow propagation therein of said undesired electromagnetic energy, saidarray being positioned along one wall of said segment with thelongitudinal axis and broad walls of said sections normal to thelongitudinal axis of the waveguide segment whereby one end of each ofsaid waveguide sections of said array is coupled to said waveguidesegment through at least one aperture in said one wall; andelectromagnetic energy absorbent material positioned in each of saidwaveguide sections to absorb electromagnetic wave energy therein.

17. A frequency selective absorptive filter for removing electromagneticwave energy of predetermined frequencies and modes from a waveguidesystem wherein electromagnetic wave energy of desired fundamentalfrequency and undesired electromagnetic wave energy of higher frequencyis propagated including the combination of a waveguide segment to beinserted in the waveguide system such that electromagnetic waves thereinmust pass through said waveguide segment; an absorptive means foreliminating substantially all wave energy of all frequencies above saidpredetermined frequency from said waveguide segment without substantialattenuation of the wave energy of frequencies below said predeterminedfrequency, said means comprising an array of rectangular waveguidesections arranged with parallel longitudinal axes including at least twoseries of such sections, each of said sections having cross sectionaldimensions such that said sections are beyond cutoff for the fundamentalelectromagnetic wave energy but which allow propagation therein of saidundesired electromagnetic wave energy, the Waveguides in one of saidseries including a plurality of sets of atleast two waveguides arrangedside by side with adjacent narrow walls,the waveguides in the other ofsaid series including a plurality of wave guides, said two series beingintercalated with adjacent broad walls to form said array, said arraybeing positioned along one wall of said segment with the longitudinalaxis and broad walls of said sections normal to the longitudinal axis ofthe waveguide segment whereby one end of each of said waveguide sectionsof said array is coupled to said waveguide segment through at least oneaperture in said one wall, and electromagnetic energy absorbent materialpositioned in each of said waveguide sections to absorb electromagneticwave energy therein.

18. An absorptive filter for removing electromagnetic wave energy ofpredetermined frequencies and modes from a waveguide system whichpropagates a desired fundamental frequency and undesired higherfrequencies, comprising: a waveguide segment for insertion in thewaveguide system such that electromagnetic waves in the system must passthrough said segment; an absorptive means for eliminating substantiallyall wave energy of all frequencies above said predetermined frequencyfrom said waveguide segment without substantial attenuation of the waveenergy of frequencies below said predeterminated frequency, said meansincluding at least one Waveguide section having dimensions whichpreclude propagation of energy of said fundamental frequency therein butwhich allow propagation therein of said higher frequencies, saidwaveguide section positioned with its longitudinal aXis parallel to thedirection of propagation in said waveguide segment, said waveguidesegment and section being directly coupled by coupling aperturesperiodically disposed in the direction of wave propagation at intervalswhich are less than one-quarter wavelength at said fundamentalfrequency; and absorbent material positioned in said waveguide sectionrelatively remote from said coupling apertures to absorb the Wave energyof said higher frequencies without aifecting transmission of saidfundamental frequency through said waveguide segment.

19. An absorptive filter for removing electromagnetic wave energy ofpredetermined frequencies and modes from a Waveguide system whichpropagates a desired fundamental wave and undesired waves of higherfrequency, comprising: a waveguide segment of rectangular cross sectionfor insertion in the Waveguide system in such a manner thatelectromagnetic Waves in the system must pass through said segment; andwave energy absorbing means for eliminating substantially all waveenergy of frequencies above said pass frequency band from said mainWaveguide without substantial attenuation of the pass frequency bandWave energy including at least two absorptive periodic structurescoupled to said Waveguide segment, said absorptive structures eachincluding at least one waveguide section having cross section dimensionswhich preclude propagation of energy of said fundamental wave thereinbut which allow propagation therein of said undesired Waves, eachabsorptive periodic structure positioned adjacent a different wall ofsaid waveguide segment with the longitudinal axes of the waveguidesections thereof parallel to the direction of propagation in saidwaveguide segment, each said Waveguide segment and section beingdirectly coupled by coupling apertures periodically disposed in thedirection of wave propagation at intervals which are small in comparisonwith the wavelength of said fundamental wave, and absorbent materialpositioned in each said waveguide sections relatively remote from thecoupling apertures to absorb said undesired waves without affectingtransmission of said fundamental wave through said waveguide segment.

26*. An absorptive filter for removing electromagnetic wave energy ofpredetermined frequencies and modes from a waveguide system propagatinga fundamental Wave and spurious waves of higher frequency comprising: afirst cylindrical waveguide segment for insertion in said system forreceiving said fundamental and spurious waves; a second cylindricalWaveguide segment coaxially disposed within said first segment, saidsecond segment having a diameter by which it is above cutoff for saidfundamental Wave but is below cutoff for said spurious waves; aplurality of coupling apertures formed in spaced relation along saidsecond segment for coupling said spurious waves into said secondsegment; and electromagnetic energy absorbent material positioned withinsaid second segment between said spaced apertures for absorbing saidspurious waves Without substantially affecting said fundamental wave.

References Cited UNITED STATES PATENTS 2,866,595 12/1958 Marie 333-732,869,085 1/1959 Pritchard et al. 333-73 2,877,437 4/1959 Farr et al333-81 2,961,619 11/1960 Breese ct al. 333-10 HERMAN KARL SAALBACH,Primary Examiner. R. F. HUNT, M. NUSSBAUM, Assistant Examiners.

1. A WAVEGUIDE FILTER FOR TRANSMITTING FIRST ELECTROMAGNETIC WAVESHAVING PREDETERMINED FUNDAMENTAL FREQUENCY AND FOR ABSORBING SECONDELECTROMAGNETIC WAVES OF HIGHER FREQUENCY, COMPRISING: AN ELONGATEDWAVEGUIDE SEGMENT FOR TRANSMITTING THERETHROUGH SAID FIRSTELECTROMAGNETIC WAVE; A WAVE ENERGY ABSORBING MEANS FOR ELMINATINGSUBSTANTIALLY ALL WAVE ENERGY OF ALL FREQUENCIES ABOVE SAIDPREDETERMINED FREQUENCY FROM SAID WAVEGUIDE SEGMENT WITHOUT SUBSTANTIALATTENUATION OF THE WAVE ENERGY OF FREQUENCIES BELOW SAID PREDETERMINEDFREQUENCY, SAID MEANS COMPRISING A PLURALITY OF WAVEGUIDE SECTIONSHAVING CROSSSECTIONAL DIMENSIONS PREVENTING PROPAGATION OF SAID FIRSTELECTROMAGNETIC WAVES AND ALLOWING PROPAGATION THEREIN OF SAID SECONDELECTROMAGNETIC WAVES, A PLURALITY OF APERTURES COUPLING SAID STRUCTURETO SAID SEGMENT, EACH APERTURE DIRECTLY COUPLING SAID SEGMENT TO ONE OFSAID SECTIONS FOR COUPLING SAID SECOND ELECTROMAGNETIC WAVES FROM SAIDSEGMENT TO SAID WAVEGUIDE SECTIONS SAID APERTURES BEING SPACED APARTALONG THE LENGTH OF SAID SEGMENT AT INTERVALS WHICH ARE LESS THANONE-QUARTER WAVELENGTH OF SAID FIRST WAVES; AND WAVE ABSORBENT MATERIALIN SAID WAVEGUIDE SECTIONS TO DISSIPATE SAID SECOND ELECTROMAGNETICWAVES, SAID ABSORBENT MATERIAL BEING DISPOSED SUFFICIENTLY REMOTE FROMSAID APERTURES TO AVOID SUBSTANTIAL INTERCEPTION OF SAID FIRSTELECTROMAGNETIC WAVES.